ath9k: remove the virtual wiphy debugfs interface

ath9k: remove the virtual wiphy debugfs interface

It does not make much sense to keep the current virtual wiphy implementation
any longer - it adds significant complexity, has very few users and is still
very experimental. At some point in time, it will be replaced by a proper
implementation in mac80211.

By making the code easier to read and maintain, removing virtual wiphy support
helps with fixing the remaining driver issues and adding further improvements.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>